One of the most intriguing and elusive phenomena in nature is the asymmetry with which the weak nuclear force acts on matter and antimatter. Certain subnuclear processes change their properties if matter is replaced by antimatter and right is exchanged with left. This phenomenon, known as CP violation, is not only conceptually compelling but also provides an avenue to extend the standard model of elementary particles, which is known to be incomplete.
B mesons are particularly suitable for investigating CP violation. They consist of a light quark — similar to those forming ordinary nuclei — bound to a much heavier bottom quark, roughly a thousand times more massive. A promising channel is the decay of the neutral B0 meson into two neutral pions (π0π0), a rare process occurring only once in about a million decays. Its identification is challenging because neutral pions promptly convert into photons, whose directions are difficult to measure with high precision.
Belle II, located at the KEK laboratory in Japan, is currently the only experiment capable of studying these decays. Installed at the interaction point of the SuperKEKB electron–positron collider, it produces thousands of B mesons per second. Recently, the Belle II group at INFN Trieste completed an analysis reconstructing 125 of these rare decays from hundreds of millions of collisions, using artificial intelligence techniques and advanced statistical models. This work yielded the most precise determinations to date of both the branching fraction of B0 → π0π0 and the overall difference between the decay probabilities of particles (B0) and antiparticles (anti-B0).
Building on this result, the Trieste team has now developed a novel method to measure, in these decays, the time dependence of CP violation — that is, how the decay rates of matter and antimatter differ as B mesons propagate through Belle II before decaying. This time-dependent behavior is highly sensitive to possible contributions from non-standard particles.
A Quantum Strategy: Using the Twin Meson
The main difficulty is that decays into purely neutral particles (such as the four photons from B0 → π0π0) leave no measurable tracks at the decay point, making direct time-dependent studies impossible. The proposed solution is to exploit the meson’s quantum partner.
At Belle II, B mesons are always produced in entangled particle–antiparticle pairs. As a result, their dynamical properties — including decay time — are correlated, akin to two dancers moving in synchrony. By measuring the decay time of the partner meson, which often decays into easily identifiable final states, one gains indirect information about the otherwise “invisible” B0 → π0π0 decay, and thus about the time evolution of CP violation. This strategy is made feasible only by the precision of the Belle II detector and the unique capabilities of the SuperKEKB accelerator, both representing the state of the art in the field.
